$inc is great to increment an existing field by a given amount, however for a newly created document you should be able to set the initial value for the field, rather than to assume it should be the same as the increment value.

Change

{ $inc: { <field1>: <amount1>, <field2>: <amount2>, ... } } 

To

{ $inc: { <field1>: {start:<value1>, next:<amount1},
             <field2>:  {start:<value2>, next:<amount2}, ... } } 

This need became apparent when using upsert = true on an update with a field that is normally incremented if it exists, but should have a user defined starting value when it does not.